Wheeler's delayed choice experiment
Glossary · History
Wheeler's delayed choice experiment is a thought experiment in quantum physics proposed by John Archibald Wheeler in 1978. The results Wheeler predicted have since been confirmed by actual experiment.
Wheeler's experiment is a variation on the famous double-slit experiment. In Wheeler's version, the method of detection used in the experiment can be changed after a photon passes the double slit, so as to delay the choice of whether to detect the path of the particle, or detect its interference with itself. Since the measurement itself seems to determine how the particle passes through the double slits – and thus its state as a wave or particle – Wheeler's experiment has been useful in trying to understand certain strange properties of quantum particles. Several implementations of the experiment from 1984-2007 showed that the act of observation ultimately determines whether the photon will behave as a particle or wave, verifying the unintuitive results of the thought experiment.
Wheeler's experiment consisted of a standard double-slit experiment, except that the detector screen could be removed at the last moment, thereby directing light into two more remote telescopes, each one focused on one of the slits. This allowed a "delayed choice" of the observer, i.e. a choice made after the presumed photon would have cleared the midstream barrier containing two parallel slits. The two telescopes, behind the (removed) screen could presumably "see" a flash of light from one of the slits, and would detect which path the photon traveled.
According to the results of the double slit experiment, if experimenters do something to learn which slit the photon goes through, they change the outcome of the experiment and the behavior of the photon. If the experimenters know which slit it goes through, the photon will behave as a particle. If they do not know which slit it goes through, the photon will behave as if it were a wave when it is given an opportunity to interfere with itself. The double-slit experiment is meant to observe phenomena that indicate whether light has a particle nature or a wave nature. The fundamental lesson of Wheeler's delayed choice experiment is that the result depends on whether the experiment is set up to detect waves or particles.
Implications of the experiment
The conventional double-slit experiment shows that determining which path a particle takes prevents the interference pattern from forming. To avoid the notion that the photon somehow "knows" when the "other" slit is open or closed (or is being watched), Wheeler suggested 'detecting' which slit the photon used only long after it passed through the slits. Wheeler asked what happens when a single photon, presumably already determined to get detected as part of a two-slit interference pattern, suddenly gets detected in a path coming from only one slit. Does the interference pattern then disappear?
In terms of the traditional double-slit apparatus, the Wheeler delayed choice experiment is to put telescopes that are pointed directly at each of the two slits behind the removable detector wall. If the photon goes through telescope A it is argued that it must have come by way of slit A, and if it goes through telescope B it is argued that it must have come by way of slit B.
Wheeler planned a thought experiment in which two ways of observing an incoming photon could be used, and the decision of which one to use could be made after the photon had cleared the double-slit part of the apparatus. At that point a detection screen could either be raised or lowered. If the detection screen were to be put in place, Wheeler fully expected that the photon would interfere with itself and (if many more photons were permitted to follow it to the screen) would form part of a series of fringes due to interference. If, on the other hand, the detection screen were to be removed, then:
Sufficiently far beyond the region of the plate, the beams from upper and lower slits cease to overlap and become well separated. There place photodetectors. Let each have an opening such that it records with essentially 100 percent probability a quantum of energy arriving in its own beam, and with essentially zero probability a quantum arriving in the other beam.
In that case, he argues, "one of the two counters will go off and signal in which beam — and therefore from which slit — the photon has arrived."
Wheeler's astronomical experiment
In a response to the argument that at short distances interactions at the screen with slits in it might be compromised by "knowledge" of events that occur at the location of the detector screen, Wheeler is reported to have come up with a more elaborate thought experiment. Wheeler suggests that one may imagine a more extraordinary scenario wherein the scale of the experiment is magnified to astronomical dimensions: a photon has originated from a star or even a distant galaxy, and its path is bent by an intervening galaxy, black hole, or other massive object, so that it could arrive at a detector on earth by either of two different paths.
The thought experiment assumes that the emitter of the photon is so positioned that the two paths are equal. If experimenters observe the single photon with a detector screen, e.g., a photographic plate or other imaging device (as in the original experiment), they should see it as part of an interference pattern (to be filled out by additional incoming photons), but if they instead use two telescopes focused to either side of the black hole they may expect to observe the photon only in one of them.
Some interpretations of Wheeler's thought experiment are premised on the belief that interference will indeed occur between the two images, and the crux of the experiment lies in determining whether identifying photons as coming from one referred image or the other will make a difference in experimental outcomes. Experimenters are already gathering light from one referred image (one pathway) by means of one telescope, and they can add light that has come by the other pathway by means of the other telescope.
If experimenters keep the two telescope images separate physically, then they ought not to expect any kind of interference fringes or other "spooky" behavior. And it is known that some photons must have reached earth via each pathway.
On the other hand, if experimenters project the images from the two telescopes onto the same spot on a detection screen and they move the images with respect to each other to change their phase relationship so that they can get cancellation in some areas and reinforcement in another area, they will then get an interference pattern, and will have demonstrated that this experiment is another version of the double-slit experiment.
There appears to be a problem, however. It may be claimed that one knows which series of waves have come by path A and which photons have come by path B, and that one has that knowledge because the photons have been physically fenced in by the tubes out of which the telescopes are constructed. However, once experimenters merge the two images on the detection screen, one can no longer know that a photon that lights up a certain spot on the detection screen has come through telescope A or through telescope B. So they have abandoned that information by mixing the two streams.
There is one more possibility, as indicated in a diagram above. If interference is actually thwarted, then photons should be found only at the position of the two primary maxima. Suppose that experimenters project the images onto two separate detection screens. That should give them a situation analogous to the one where they were viewing a distant light source with only one slit open. In the physics laboratory there are some diffraction effects due to light's having been put through a narrow opening, but not the broad band that is known as the interference fringe.
If interference between the images brought in via two telescopes does appear, experimenters ought to see dimmer images at the secondary, tertiary, etc. maxima predicted for interference effects. They should not expect to see the same range and clarity of secondary images (if any at all) with one telescope capped off. What occurs in this case is again a matter for empirical study to determine.
The idea behind some interpretations of the Wheeler experiment is that it might be possible to determine which side of a double-slit experiment a photon traveled through without destroying the interference pattern that occurs when the two versions of its probability wave interact on the detector screen of the typical double-slit experiment. Another view is that whether interference fringes are noted or not depends not on anything that happened between the distant star and earth. Instead, it depends entirely on what form the observation or the measurement of the photon or photons takes. Looking for a photon on one path or the other will produce the observation of one photon at a single point by a telescope aimed in a certain direction. Looking for an interference pattern by merging the beams coming through both paths will produce interference fringes.
The first experimental realization of a delayed-choice experiment was carried out by Carroll Alley, Oleg Jakubowicz, and William Wickes in 1984 at the University of Maryland, cited by John Wheeler in his autobiography. Wheeler gave a seminar on the delayed-choice idea at the U of M in 1979 that inspired Alley and Wickes, who knew Wheeler well from their days as students and faculty at Princeton University, to work with Jacubowicz to translate the gedanken-experiment (thought experiment) into a laboratory test. The experiment confirmed the quantum mechanical predictions for a photon's behavior in an interferometer that is randomly reconfigured or not after the photon passes through the initial beam-splitter.
Most recent experiment
In 2000, Yoon-Ho Kim, et al., reported success in their delayed choice quantum eraser experiment, a variation that combines Wheeler's delayed choice experiment with a quantum eraser experiment, so that the choice to observe the photon or not is done after it hits the detector.
Another Quantum eraser experiment was done in 2002 by S. P. Walborn, M. O. Terra Cunha, S. Padua, and C. H. Monken.
Researchers with access to radio telescopes originally designed for SETI research have pointed to the possibility, and have explicated the practical difficulties, of conducting the Wheeler experiment with actual stellar objects.
- Vincent Jacques et al., Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment, Science Vol. 315. no. 5814, pp. 966 - 968 (2007). Preprint available at http://arxiv.org/abs/quant-ph/0610241v1
- John Archibald Wheeler, "The 'Past' and the 'Delayed-Choice Double-Slit Experiment'," pp 9–48, in A.R. Marlow, editor, Mathematical Foundations of Quantum Theory, Academic Press (1978)
- John Archibald Wheeler and Wojciech Hubert Zurek, Quantum Theory and Measurement (Princeton Series in Physics)
- John D. Barrow, Paul C. W. Davies, and Jr, Charles L. Harperm Science and Ultimate Reality: Quantum Theory, Cosmology, and Complexity (Cambridge University Press) 2004
- Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, Academic Press, 1978.
- C.O. Alley, O. Jakubowicz, W.C. Wickes, Results of the Delayed Random Choice Quantum Mechanics Experiment With Light Quanta and Proposal of a New Type of EPR Experiment Using Light Quanta Produced by a Nonlinear Optical Process, In Namiki M et al. (eds), Proceedings of the 2nd International Symposium: Foundations of Quantum Mechanics in the Light of New Technology. Tokyo, Physical Society of Japan, 1987, pp. 36-52
- Cho, Adrian. After a Short Delay, Quantum Mechanics Becomes Even Weirder. ScienceNOW Daily News. 16 February 2007
- Castelvecchi, Davide. Tight Deadline. Photons: Decide what to do – and do it yesterday. Science News online. May 23, 2008
- John Archibald Wheeler, "The 'Past' and the 'Delayed-Choice' Double-Slit Experiment", in Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, p. 13
- Source for this experiment in Wheeler's own writing has not been traced yet. Dr. John Cramer indicates that Wheeler offered the idea in response to criticism of a proposed experiment on a smaller scale. (Personal communication.)
- Geons, Black Holes & Quantum Foam: A Life in Physics, by John Archibald Wheeler with Kenneth Ford, W.W. Norton & Co., 1998, p. 337
- Jacques, Vincent; et al. (2007). "Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment". Science 315: 966–968. arXiv:quant-ph/0610241v1. Bibcode:2007Sci...315..966J. doi:10.1126/science.1136303. PMID 17303748.
- Quantum Astronomy (IV): Cosmic-Scale Double-Slit Experiment